doc: sysbuild: Update documentation about BUILD_ONLY

   )

As a result, ``my_sample`` will be built as part of the sysbuild build invocation,

but neither ``west flash`` nor ``west debug`` will be aware of this application.

but it will be excluded from the default image sequence used by ``west flash``.

Instead, you may use the outputs of this domain for other purposes - for example,

to produce a secondary image for DFU, or to merge multiple images together.

You can also replace ``TRUE`` with another boolean constant in CMake, such as

a Kconfig option, which would make ``my_sample`` conditionally build-only.

.. note::

   Applications marked as build-only can still be flashed manually, using

   ``west flash --domain my_sample``. As such, the ``BUILD_ONLY`` option only

   controls the default behavior of ``west flash``.

Zephyr application configuration

================================